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Ok Mark. I've been doing some reading and some back of the envelope calculations. I now believe that the prohibitions about which I was reading in the data sheets apply primarily to oscillators and RF conditions. Examination of the transconductance curves leads me to believe that this tube can be used in a CCTA configuration, with fixed bias, without thermal runaway problems.

So the circuit will look something like this.

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6J6 Preamp Fixed-Bias Schematic.jpg

Note that these component values are only preliminary at this time. I will need a few days to prototype and test to make sure it works ok. I also have a way to generate the bias voltages without the addition of much circuitry to a normal power supply.

I'll let you know when I have some test data to confirm my calculations.

Matt, would you please supply a bias voltage, Vb. I'm between projects and has a pile of parts to go.

After some conflab with others the caps are probably metalised polypropylene. Even better.

A nearby friend in audio DIY crime did some testing for me on Lagoon. This is a 4S 12AU7 single stage tube preamp. The results are good.

Distortion is quite low at 0.05% with the second harmonic predominant as expected. There was 50hz and 100hz hum there but low down and not audible in practice.

Frequency response was from less then 30hz to over 20K which is all the test gear goes to. BUT this was into a crippling 15K resistive load.

This leads me to question that these single stage preamps will only work into high impedance load, typically 250K!. There is an online impedance calculator in which you enter tube type, plate load resistor Rp and cathode resistor Rk and in bypassed or not. The calculator claims the output impedance should be about 13K! A lot lower than I have ever thought.

To test this out we maxed the output of the preamp just prior to clipping (30V) then loaded it down until we got half the output voltage - about 13V. This occurred at a load of 15K!!! I maybe be wrong but this indicates the output impedance is 15K. Right or wrong.

Very pleasing results all round and now I know this 4S preamp will drive SS power amps which have a input impedance of better than 15K (most will) and need less than a 13V swing - ALL WILL. Not only that the preamp sounds excellent.

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LagoonS.jpg

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LagoonFreqResS.jpg

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This leads me to question that these single stage preamps will only work into high impedance load, typically 250K!.

This is true, when the volume control is on the input rather than the output. The output impedance of the 4S using a 12AU7 is approximately 11.5kΩ. It's only when we add the volume control that it looks like this:

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Screen Shot 2018-08-12 at 8.49.25 AM.png

This is the 4S using a 12AU7 and a 250kΩ volume pot on the output.

Now output impedance is one part of the equation but load line rotation is the other. When we couple the preamp to a stage with low input impedance, then this impedance loads the tube in parallel with the load resistor. So if it is coupled to a stage with a 20kΩ input impedance, the the AC load line on the tube would really only be ≈16.6kΩ. This won't have a lot of impact for small signals (as the measurements demonstrate) but for larger inputs, the distortions can become large.

This is why, when coupling to a low impedance amplifier, I still recommend a buffer to protect the proper operation of both the preamp and the amplifier.

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Thanks Matt, all makes sense. I have used a two stage tube preamp to drive SS amps and always thought it OK. The two stage didn't have a cathode follower out but did have global feed back. Can global feed back help with managing load?

I was frustrated by the fact I couldn't find a suitable, deep enough enclosure for this preamp. The tube would have been hidden. Then thought if I flip the socket the tube will be on the bare strip side and can stand proud of the board. Also a cheaper shallower case.

I was about to make another brd. and thought why not just flip the socket on this one. Here is the result.

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